To power off system, and begin the shutdown process: Hold the power button for < 0.5 seconds. If you don't have a button, you can also close the pins BP PIN C and BP PIN NO, with your favorite electrically conductive metal (eg: tweezers).

NB: Avoid holding the power button for longer than 2 seconds, as this will hard power off system (same effect as pulling the power). Doing so will create file-system corruptions, during shutdown IO operations.